A breathing mask with port openings for filters as well as with an inspiration valve and with an expiration valve is known from DE 40 17 336 C1. Respirator filters can be arranged at various points of the half mask by means of a connection adapter, which can be fastened to a half mask in two preferential positions. The gas flow is sent to the mask user via the respirator filters into the mask interior by means of directional valves, which are arranged both in the area of the respirator filters and at the gas outlet of the breathing mask, and the breathing gas enters the environment via an expiration valve during the expiration phase. The directional valves comprise a valve seat and a valve membrane, which lies on the valve seat and is fastened centrally to the valve seat. The valve membrane is lifted off from the valve seat during gas flow, so that a gas flow is made possible. The drawback of the prior-art valve construction is that the flow resistance is relatively high. The length of the membrane section that can be deflected by the gas flow is limited by the diameter of the valve seat due to the central fastening of the valve membrane.
Directional valves for breathing masks are known, which have a valve membrane fastened to the valve seat on one side and can be lifted off from the valve seat in a flap-like manner. Such a directional valve appears, for example, from U.S. Pat. No. 6,047,698. A valve membrane cut to a rectangular shape lies on a flat valve seat and is pressed by a fixing lug fastened to the edge of the valve bonnet against the valve seat. Even though a markedly lower flow resistance can be obtained with this valve construction compared to a directional valve with centrally fastened valve membrane, the leakage values are nevertheless higher because the valve membrane is fastened on one side, and the valve membrane may remain stuck to the inside of the valve bonnet due to adhesive forces in case of a possible condensation of moist expired air, as a result of which the closing operation of the directional valve may be delayed or the closing operation may fail to occur altogether.